Why is Europe cold?

Longer answer: it's the NAO, and no it doesn't mean global warming is a hoax.

Winter is always an interesting time for climate researchers. The oceanographer in me has always been fascinated with how the ocean's memory (encoded as the distribution of temperature and salinity) has a tendency to remember winter and forget summer. As someone interested in climate change and how it is perceived and portrayed, winter is when the crazy comes out. It's easy for people to conclude the earth might be warming when they're baking in a July heat wave. It's a much harder sell when you're shoveling snow.

The recent spate of cold weather in Europe is a great example. Last winter, Europe was colder than it had been in recent memory, and this year seems off to a similar start. Winter weather in Europe is strongly correlated with the North Atlantic Oscillation (NAO) Index. The NAO is essentially a measure for the size and strength of the cold, high pressure region centered over Iceland and the warm, low pressure region centered over the Azores. During the "positive" state of the NAO, the low is lower, the high is higher, and the high pressure area expands northward. This brings mild weather to most of Europe and pushes the track of winter storms over Scandinavia. When the NAO is in its "negative" state, the low pressure area expands southward, bridging colder air over Europe. For reasons that are still unclear (at least to me), the NAO can often get locked into one of these states, with positive or negative conditions persisting for several months. This is what happened last year--the monthly NAO was strongly negative from October 2009 to now. This year looks to be headed in the same direction, and the recent strongly negative NAO conditions are the proximal explanation for Europe's cold spell.

Whether the negative NAO will stick around and keep Europe frozen all winter is anyones guess. Regardless, it has almost nothing to do with climate change. The main global warming hypothesis, namely, that increased CO2 in the atmosphere causes global mean temperatures to rise, is fundamentally about the total amount of heat on the planet. Europe can be cold, but if the rest of the world is warmer, the global mean temperature will go up. This is exactly what happens with the NAO. Cold conditions in Europe are often associated with warmer conditions in the Canadian Maritimes, and often in New England. Based on the weather here in Portland (still haven't dug the snow boots out of the garage, and Walt is still wearing shorts), we could write an article about the dire consequences from global warming. Of course, this would be equally wrong as some of the stories coming from Europe. Before anyone declares that climate change is not occurring because it snowed in London, they should do their homework. If it's also colder in New York, Tokyo, Mumbai, Sydney, Cape Town, and Palmer Station, then they might have something.